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From: Jurko Gospodnetić (jurko.gospodnetic_at_[hidden])
Date: 2008-05-25 12:18:45

   Hi Volodya.

>> P.S.
>> There is a different topic of whether to maintain that folder as we
>> do now or perhaps use just use a SHA-1 hash of the used property set and
>> not organize the build folders hierarchically. This would help avoid the
>> problem with build folder paths becoming too long for some projects and
>> hitting the OS path size limit.
> I always wanted to use signature of build properties to determine if a target
> has to be rebuilt, but never got around to implement that.
> Technically, there are two approaches
> 1. Leave current directory naming intact, but compute and store SHA-1
> signatures of properties or maybe build command. Rebuild the target
> is the signature has changes. This will make sure a target is rebuild
> when you add a define, or cxxflags, or change compiler command. OTOH,
> it's not always desirable to rebuild everything in such case, so we
> might want some "just update signatures" option.
> 2. Use SHA-1 for directory names. This is almost the same, with the
> added benefit of making directory names shorter. But to make
> "just update signatures" possible, we'd need to compute the SHA-1 of
> only the properties that now show up in path, and separately compute SHA-1
> of all properties for each generated file.

   As I see it, Boost Build builds a single target into separate build
folders depending the properties used for it. This is done so that
targets built with matching properties would not overwrite each other
and you can more easily use different builds of the same target in the
same project, i.e. without having to tweak your Jamfiles to make such a
thing possible.

   Now, forcing targets to be rebuilt for properties that do not have
their corresponding build folder seems contradictory to this original idea.

   The only general solution I can think of that supports multiple
builds of the same target using different properties within the same
project/build is to use build folders whose names are calculated based
on the non-incidental properties used to build targets contained in
those folders. For example, we can use SHA-1 hashed together with a
description file in that folder listing the exact property values used.

   Good sides:
     * Build folders have short paths - therefore avoiding the long path
problems encountered on larger projects.
     * Projects with targets built using multiple property sets do not
require any special handling for those files as they all work correctly
out of the box.
     * Free features like cxxflags may be used when deciding which build
folder to use. This means that solutions like ad-hoc compiler option
tweaks will work cleanly instead of having to be implemented using
separate Boost Build features for every such solution.

   Bad sides:
     * Build folders become more clumsy to locate by humans when they
know the exact property set used.

   As I see it the bad side does not seem that bad to me as I rarely
ever need to go trudging through the Boost Build folders looking for a
specific built target. If needed, the build system could also be
upgraded so it can be asked: 'where will you place this target?'.

   Also, it would be relatively straightforward to add support for
explicitly named build folders for a specific property set.

   As far as free features affecting the build go - one can say that
they should be modeled as separate features if they are important, but
one can also argue that this is just extra overhead imposed by the build
system that should be able to directly understand that 'typically'
specifying different cxxflags should cause targets to be built in
different folders (or at least under different names). Alas, in real
life this is a moot area since there really are such free feature values
that are incidental (i.e. do not affect the built targets) and there are
those that are not. Also, in real life, projects that require building
the same target with multiple properties are minority. All things
considered, I guess the build system should make the conservative choice
and consider all free features as non-incidental unless explicitly
tagged as such, which the current implementation does not.

   The current solution of using separate folder per property has the
following good and bad sides with respect to the solution presented above:

   Good sides:
     * Easy for a human to navigate the build folder tree.

   Bad sides:
     * Free features must always be incidental as Boost Build can not
construct folder names directly, mostly due to different OS restrictions
on what characters can be included in folder names.
     * Build folder tree can easily become very large and break OS
imposed limits on the maximum length (e.g. 260 on Windows unless using
tricks in BJam code and any external build tools as well, which might
not even be possible).

   I am attaching the version of this posting as a new ticket in the
Boost Build trac (
so it does not get lost.

   Best regards,
     Jurko Gospodnetić

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